Methods and devices for enhancing mobility of charge carriers

ABSTRACT

Methods and devices for enhancing mobility of charge carriers. An integrated circuit may include semiconductor devices of two types. The first type of device may include a metallic gate and a channel strained in a first manner. The second type of device may include a metallic gate and a channel strained in a second manner. The gates may include, collectively, three or fewer metallic materials. The gates may share a same metallic material. A method of forming the semiconductor devices on an integrated circuit may include depositing first and second metallic layers in first and second regions of the integrated circuit corresponding to the first and second gates, respectively.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 13/856,325, filed Apr. 3, 2013, which is incorporated herein byreference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to methods and devices for enhancingmobility of charge carriers. Some embodiments of the present disclosurerelate particularly to integrated circuits with metal-gate semiconductordevices, in which the channel of one type of metal-gate semiconductordevice is compressively strained, and the channel of another type ofmetal-gate semiconductor device is tensilely strained.

2. Description of the Related Art

Straining the channel of a semiconductor device may improve the device'sperformance. Some semiconductor devices, such as transistors, have achannel through which charge carriers move when the semiconductor deviceis activated. The mobility of charge carriers in the device's channelmay be an important factor in determining the device's performance. Forexample, the switching speed and/or drive strength of a semiconductordevice may depend on the mobility of charge carriers in the device'schannel. Straining the channel of a semiconductor device may enhance themobility of charge carriers in the channel (relative to the mobility ofcharge carriers in an unstrained channel), thereby improving thedevice's performance (e.g., switching speed or drive strength). Forexample, the mobility of holes (charge carriers for some types ofsemiconductor devices, such as n-channel MOSFETs) may be enhanced in atensilely strained channel. A tensilely strained channel may be deformed(e.g., stretched) by the application of a tensile stress. As anotherexample, the mobility of electrons (charge carriers for some types ofsemiconductor devices, such as p-channel MOSFETs) may be enhanced in acompressively strained channel. A compressively strained channel may bedeformed (e.g., compressed) by the application of compressive stress.

Techniques for fabricating semiconductor devices with strained channelsare known. Strain may be induced in a silicon substrate by growing thesilicon substrate on top of another crystalline substrate with adifferent lattice. For example, tensile strain may be induced by growinga silicon substrate on top of silicon-germanium (SiGe), which has alarger lattice than silicon and therefore applies tensile stress to thesilicon lattice. As another example, compressive strain may be inducedby growing a silicon substrate on top of a silicon carbide (e.g., SiCP),which has a smaller lattice than silicon and therefore appliescompressive stress to the silicon lattice. However, with this technique,it may be difficult to induce compressive strain in some portions of thesilicon (e.g., for PFET channels) and to induce tensile strain in otherportions of the silicon (e.g., for NFET channels).

A strained channel may be formed by implanting materials into a siliconsubstrate to change the lattice in regions of the substrate (e.g., inchannel regions). For example, implantation may be used to form atensilely strained silicon carbide (e.g., SiCP) channel region, becausethe larger lattice of the silicon substrate applies tensile stress tothe smaller lattice of the silicon carbide channel. As another example,implantation may be used to form a compressively strainedsilicon-germanium (e.g., SiGe) channel region, because the smallerlattice of the silicon substrate applies compressive stress to thelarger lattice of the silicon-germanium channel. However, this techniquemay require very low temperatures during the fabrication process (e.g.,−60 C), and may exacerbate short-channel effects (SCE).

A strained channel may also be formed by forming “liners” or “cappinglayers” on the gates of semiconductor devices. For example, a siliconnitride liner formed on the gate of a PFET may apply compressive stressto the PFET's channel, and a different silicon nitride liner formed onthe gate of an NFET may apply tensile stress to the NFET's channel.However, this technique may require additional process steps, includingchemical-mechanical polishing (CMP).

When the gate of a semiconductor device includes a metallic material,the device's “metal gate” may apply stress to the channel, therebyforming a strained channel. Some metal gates may include a metallicportion and a work-function layer. The work-function layer may modulatethe gate's work function, thereby giving a process engineer control overthe device's band gap, threshold voltage, etc.

BRIEF SUMMARY

According to an embodiment, there is provided an integrated circuitincluding a first semiconductor device of a first type and a secondsemiconductor device of a second type. The first semiconductor device ofthe first type including a first gate and a first strained channel. Thefirst gate includes a first metallic portion. The first strained channelis strained in a first manner. The second semiconductor device of thesecond type includes a second gate and a second strained channel. Thesecond gate includes a second metallic material. The second strainedchannel is strained in a second manner. The first and second gatescollectively include three or fewer metallic materials.

According to another embodiment, there is provided an integrated circuitincluding a first semiconductor device of a first type and a secondsemiconductor device of a second type. The first semiconductor device ofthe first type including a first gate and a first strained channel. Thefirst gate includes a first metallic portion. The first strained channelis strained in a first manner. The second semiconductor device of thesecond type includes a second gate and a second strained channel. Thesecond gate includes a second metallic material. The second strainedchannel is strained in a second manner. A same metallic material isincluded in the first gate of the first semiconductor device and in thesecond gate of the second semiconductor device.

According to another embodiment, there is provided an integrated circuitincluding a first semiconductor device of a first type and a secondsemiconductor device of a second type. The first semiconductor device ofthe first type including a first gate and a first channel. The firstgate includes a first metallic material. The second semiconductor deviceof the second type including a second gate and a second channel. Thesecond gate includes a second metallic material. The integrated circuitfurther includes means for using the first and second gates to increasemobility of charge carriers in channels of the first semiconductordevice and the second semiconductor device, respectively. The first andsecond gates collectively include three or fewer metallic materials.

According to another embodiment, there is provided an integrated circuitincluding a first semiconductor device of a first type and a secondsemiconductor device of a second type. The first semiconductor device ofthe first type including a first gate and a first channel. The firstgate includes a first metallic material. The second semiconductor deviceof the second type including a second gate and a second channel. Thesecond gate includes a second metallic material. The integrated circuitfurther includes means for using the first and second gates to increasemobility of charge carriers in channels of the first semiconductordevice and the second semiconductor device, respectively. A samemetallic material is included in the first gate of the firstsemiconductor device and in the second gate of the second semiconductordevice.

According to another embodiment, there is provided a method of formingsemiconductor devices on an integrated circuit. A first of thesemiconductor devices has a first gate and a first strained channel, asecond of the semiconductor devices has a second gate and a secondstrained channel. The method includes depositing a first metallic layerin first and second regions of the integrated circuit corresponding tothe first and second gates, respectively. The method further includesdepositing a second metallic layer in the first and second regions ofthe integrated circuit corresponding to the first and second gates,respectively. The first and second gates collectively include three orfewer metallic materials. The first strained channel is strained in afirst manner, and the second strained channel is strained in a secondmanner.

According to another embodiment, there is provided a method of formingsemiconductor devices on an integrated circuit. A first of thesemiconductor devices has a first gate and a first strained channel, asecond of the semiconductor devices has a second gate and a secondstrained channel. The method includes depositing a first metallic layerin first and second regions of the integrated circuit corresponding tothe first and second gates, respectively. The method further includesdepositing a second metallic layer in the first and second regions ofthe integrated circuit corresponding to the first and second gates,respectively. A same metallic material is included in the first gate ofthe first semiconductor device and in the second gate of the secondsemiconductor device. The first strained channel is strained in a firstmanner, and the second strained channel is strained in a second manner.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For an understanding of some embodiments, reference will now be made byway of example only to the accompanying figures in which:

FIG. 1 shows an integrated circuit, according to some embodiments;

FIG. 2A shows a transistor 200 a with a gate 240 a that includes one ormore metallic materials, according to some embodiments;

FIG. 2B shows a transistor 200 b with a gate 240 b that includes one ormore metallic materials, according to some embodiments;

FIG. 2C shows a transistor 200 c with a gate 240 c that includes one ormore metallic materials, according to some embodiments;

FIG. 2D shows a transistor 200 d with a gate 240 d that includes one ormore metallic materials, according to some embodiments;

FIGS. 3A and 3B show a flowchart of a method of forming semiconductordevices on an integrated circuit, according to some embodiments;

FIG. 4A shows a portion of an integrated circuit 400 a after formationof source and drain diffusion regions, according to some embodiments;

FIG. 4B shows a portion of an integrated circuit 400 b after formationof source and drain diffusion regions, according to some embodiments;

FIG. 5A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last process using steps 304,306, 308, 312, 314, and 333 of the method illustrated in FIGS. 3A and3B, according to some embodiments;

FIG. 5B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 312, 314, 333, and 336 of the method illustrated in FIGS. 3Aand 3B, according to some embodiments;

FIG. 6A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 312, 314, 318, 320, 323, and 333 of themethod illustrated in FIGS. 3A and 3B, according to some embodiments;

FIG. 6B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 312, 314, 318, 320, 323, 333, and 336 of the methodillustrated in FIGS. 3A and 3B, according to some embodiments;

FIG. 7A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 312, 314, 318, 320, and 333 of the methodillustrated in FIGS. 3A and 3B, according to some embodiments;

FIG. 7B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 312, 314, 318, 320, 333, and 336 of the method illustrated inFIGS. 3A and 3B, according to some embodiments;

FIG. 8A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 324, 328, 330, 332, and 333 of the methodillustrated in FIGS. 3A and 3B, according to some embodiments;

FIG. 8B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 324, 328, 330, 332, 333, and 336 of the method illustrated inFIGS. 3A and 3B, according to some embodiments;

FIG. 9A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 324, and 333 of the method illustrated inFIGS. 3A and 3B, according to some embodiments; and

FIG. 9B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 324, 333, and 336 of the method illustrated in FIGS. 3A and3B, according to some embodiments.

For clarity, the same elements have been designated with the samereference numerals in the different drawings and, further, as usual inthe representation of integrated circuits, the various drawings are notto scale. For clarity, only those steps and elements which are useful tothe understanding of the described embodiments have been shown and willbe discussed.

DETAILED DESCRIPTION

Some conventional techniques for fabricating metal-gate semiconductordevices may produce integrated circuits containing tensilely strainedchannels or compressively strained channels, but not both. Someconventional techniques for fabricating metal-gate semiconductor devicesmay use a large number of metals and/or processing steps. Applicant hasrecognized and appreciated the need for a method of fabricatingmetal-gate semiconductor devices using a small number of processingsteps and/or metallic materials, such that the resulting integratedcircuit includes both tensilely strained channels and compressivelystrained channels.

According to an embodiment, an integrated circuit may includesemiconductor devices of first and second types. For example, theintegrated circuit may include NFETs and PFETs. A semiconductor deviceof the first type (e.g., an NFET) may include a gate and a tensilelystrained channel. A semiconductor device of the second type (e.g., aPFET) may include a gate and a tensilely strained channel. The gates mayinclude one or more metallic materials.

In some embodiments, the gates of the two types of semiconductor devicesmay collectively include three or fewer metallic materials. In someembodiments, a same metallic material may be included in the gates ofthe two types of semiconductor devices.

In some embodiments, the gate of the first type of semiconductor device(e.g., NFET) may apply tensile stress to the channel of the first typeof semiconductor device. In some embodiments, the gate of the secondtype of semiconductor device (e.g., PFET) may apply compressive stressto the channel of the second type of semiconductor device.

In some embodiments, the gates of either or both types of semiconductordevices may include work-function layers. In some embodiments,properties of a device's work-function layer (e.g., the thickness of thework-function layer, the material(s) included in the work-functionlayer, or the power provided to a cathode used for cathodic arcdeposition of the work-function layer) may modulate the stress appliedto the device's channel by the device's metal gate. For example,properties of the work-function may determine whether the stress appliedto the channel by the gate is compressive or tensile. As anotherexample, properties of the work-function layer may determine themagnitude of the stress applied to the channel by the gate.

The features described above, as well as additional features, aredescribed further below. These features may be used individually, alltogether, or in any combination, as the technology is not limited inthis respect.

FIG. 1 shows an integrated circuit, according to some embodiments.Integrated circuit 100 of FIG. 1 includes two transistors 110 and 150.In some embodiments, transistors 110 and 150 may be MOSFETs (metal-oxidesemiconductor field effect transistors). In some embodiments, transistor110 may be an n-channel MOSFET (also referred to as an “NMOS FET” or“NFET”). In some embodiments, transistor 150 may be a p-channel MOSFET(also referred to as a “PMOS FET” or “PFET”).

In the embodiment of FIG. 1, the bodies of transistors 110 and 150 areformed in regions 112 and 152, respectively, of substrate 102. In someembodiments, substrate 102 may include silicon, silicon germanium,silicon carbide, and/or other material(s) known to one of ordinary skillin the art or otherwise suitable for fabricating semiconductor devices.In some embodiments substrate 102 may be a p-substrate, region 112 maybe a portion of the p-substrate, and region 152 may be an n-well formedin the p-substrate. In some embodiments, substrate 102 may be an-substrate, region 112 may be a p-well formed in the n-substrate, andregion 152 may be a portion of the n-substrate. In some embodiments,substrate 102 may be a bulk substrate, a silicon-on-insulator (SOI)substrate, a strained-silicon-direct-on-insulator (SSDOI) substrate, astrained heterostructure-on-insulator (HOI) substrate, or any other typeof substrate known to one of ordinary skill in the art or otherwisesuitable for fabricating semiconductor devices.

In some embodiments, transistors 110 and 150 may be fully or partiallyisolated from each other using any technique known to one of ordinaryskill in the art or otherwise suitable for isolating semiconductordevices, including but not limited to shallow trench isolation (STI).

In the embodiment of FIG. 1, transistor 110 includes a gate 118, asource diffusion region (also referred to as a “source” or “sourcediffusion”) 114, a drain diffusion region (also referred to as a “drain”or “drain diffusion” 116), a channel 122, and a body. In someembodiments, the source and drain diffusions may be doped (e.g.,heavily-doped) p-type diffusion regions. In some embodiments, source 114and or drain 116 may be raised. A raised source or drain may be formedusing any technique known to one of ordinary skill in the art orotherwise suitable for forming a raised source or drain, including butnot limited to etching a portion of substrate 102 and/or siliciding aportion of a diffusion region. As described above, the body oftransistor 110 may be formed in region 112 of substrate 102, which maybe an n-doped silicon region. Channel 122 of transistor 110 may occupyportions of substrate 102 between source 114 and drain 116 under gate118. Gate 118 may include, for example, an insulating layer formed onsubstrate 102 and a material portion formed on the insulating layer. Insome embodiments, the material portion may be formed, for example, frompolysilicon, one or more metallic materials, and/or any materials knownto one of ordinary skill in the art or otherwise suitable for forming agate.

In some embodiments, gate 118 may include or be partially or fullycovered by a spacer layer, a liner, a capping layer, and/or any othertype of ‘gate-covering layer.’ A gate-covering layer may be formed nearthe gate of a transistor (e.g., over the gate and/or adjacent to thesidewalls of the gate) by means known to one of ordinary skill in theart or otherwise suitable for forming a gate-covering layer, includingbut not limited to deposition and photolithographic patterning of agate-covering material. In some embodiments, a gate-covering layer mayinclude a nitride and/or an oxide, such as silicon nitride (SiN) orsilicon oxide (SiO). In some embodiments, a gate-covering layer mayinsulate the gate from other portions of the integrated circuit,facilitate a self-aligning transistor fabrication process, apply stressto the transistor channel, etc.

In the embodiment of FIG. 1, transistor 150 includes a gate 158, asource diffusion region 154, a drain diffusion region 156, a channel162, and a body. In some embodiments, the source and drain diffusionsmay be doped (e.g., heavily doped) n-type diffusion regions. Asdescribed above, the body of transistor 150 may be formed in region 152of substrate 102, which may be a p-doped silicon region. Channel 162 oftransistor 150 may occupy portions of substrate 102 between source 154and drain 156 under gate 158. Gate 158 may include, for example, aninsulating layer formed on substrate 102 and a material portion formedon the insulating layer. In some embodiments, the material portion maybe formed, for example, from polysilicon, one or more metallicmaterials, and/or any materials known to one of ordinary skill in theart or otherwise suitable for forming a gate. In some embodiments, gate158 may include or be partially or fully covered by gate-covering layer.

In the embodiment of FIG. 1, channels 122 and 162 are strained. In someembodiments, channels 122 and 162 may be strained in different ways. Forexample, channel 122 may be tensilely strained, and channel 162 may becompressively strained. In some embodiments, channel 122 may betensilely strained in a horizontal direction 124 (e.g., stretched in adirection roughly parallel to a surface of the substrate, such as adirection extending between source 114 and drain 116), and/or tensilelystrained in a vertical direction 126 (e.g., stretched in a directionroughly orthogonal to a surface of the substrate, such as a directionextending between gate 118 and substrate 102). Likewise, channel 162 maybe compressively strained in a horizontal direction 164 (e.g.,compressed in a direction roughly parallel to a surface of thesubstrate, such as a direction extending between source 154 and drain156), and/or compressively strained in a vertical direction 166 (e.g.,compressed in a direction roughly orthogonal to a surface of thesubstrate, such as a direction extending between gate 158 and substrate102).

Although MOSFETS are shown in the example of FIG. 1, embodiments are notlimited in this regard. Embodiments may include (or be used tofabricate) any semiconductor devices known to one of ordinary skill inthe art or otherwise suitable for operating with strained channels,including but not limited to diodes, other types of transistors, etc.

FIG. 2A shows a transistor 200 a with a gate 240 a that includes one ormore metallic materials, according to some embodiments. A transistorwith a gate that includes one or more metallic materials may be referredto as a “metal-gate transistor.” In some embodiments, metal-gatetransistor 200 a may be formed using a gate-last fabrication process,such as a replacement-gate fabrication process. In some embodiments,metal-gate transistor 200 a may be a MOSFET with a gate 240 a, a sourcediffusion region 208, a drain diffusion region 206, a channel 204, and abody. The body of metal-gate transistor 200 a may be formed in a regionof substrate 102. The source and drain diffusion regions may be doped(e.g., heavily doped). The doping types of diffusion regions 206 and 208and the body region may be opposite doping types. In some embodiments,channel 204 may extend between the source and drain regions under gate240 a. In the embodiment of FIG. 2A, source 208 and drain 206 are raisedto form raised source 218 and raised drain 216. However, embodiments arenot limited in this regard. In some embodiments, metal-gate transistor200 a may include a source that is not raised and/or drain that is notraised.

In the embodiment of FIG. 2A, gate 240 a of metal-gate transistor 200 aincludes a metallic portion 224. Metallic portion 224 may include, forexample, aluminum (AL), tungsten (W), copper (Cu), and/or any othermetallic material known to one of ordinary skill in the art or otherwisesuitable for functioning as a metallic portion of a gate of a metal-gatetransistor.

In the embodiment of FIG. 2A, gate 240 a of metal-gate transistor 200 aincludes a dielectric layer 222. In some embodiments, dielectric layer222 may insulate metallic portion 224 from other portions of anintegrated circuit, including but not limited to source 208 and/or drain206. In some embodiments, dielectric layer 222 may include a dielectricmaterial, such as polysilicon, a high-k dielectric material (e.g., amaterial having a dielectric constant higher than the dielectricconstant of polysilicon), and/or any other material known to one ofordinary skill in the art or otherwise suitable for insulating portionsof a gate of a metal-gate transistor. For example, dielectric layer 222may include hafnium oxide (HfO₂). In some embodiments, portions ofdielectric layer 222 may be formed over substrate 102 and under metallicportion 224. In some embodiments portions of dielectric layer 222 may beformed vertically along the sidewalls of gate 240 a.

In the embodiment of FIG. 2A, channel 204 is strained. Channel 204 maybe strained horizontally and/or vertically. In some embodiments (e.g.,in embodiments where transistor 200 a is an NFET), channel 204 may betensilely strained. In some embodiments (e.g., in embodiments wheretransistor 200 a is a PFET), channel 204 may be compressively strained.

In some embodiments, gate 240 a may apply stress to channel 204. Thestress applied by gate 240 a may contribute to the strain in channel204. For example, in some embodiments, metallic portion 224 may applystress (e.g., compressive or tensile stress) to channel 204,contributing to the strain (e.g., compressive or tensile strain,respectively) in channel 204. In some embodiments, the magnitude of thestress applied to channel 204 by gate 240 a may be less than 100 MPa,between 100 and 300 MPa, between 300 and 500 MPa, or greater than 500MPa. In some embodiments, gate 240 a of metal-gate transistor 200 a mayinclude or be fully or partially covered by a gate-covering layer 220.

FIG. 2B shows a transistor 200 b with a gate 240 b that includes one ormore metallic materials (a “metal-gate transistor”), according to someembodiments. In some embodiments, metal-gate transistor 200 b may be aMOSFET formed using a gate-last semiconductor fabrication process. Manyelements of metal gate transistor 200 b shown in FIG. 2B have alreadybeen described above with reference to FIG. 2A. For brevity, thedescription of such elements is not repeated here.

In some embodiments, gate 240 b of metal-gate transistor 200 b mayinclude, in addition to dielectric layer 222 and metallic portion 224, awork-function layer 230. In some embodiments, work-function layer 230may be formed between metallic portion 224 and dielectric layer 222. Forexample, portions of work-function layer 230 may be formed overdielectric layer 222 and under metallic layer 224. As another example,portions of work-function layer 230 may be formed between sidewallportions of metallic portion 224 and dielectric layer 222.

In some embodiments, the magnitude and/or type of stress applied tochannel 204 by gate 240 b may depend on properties of work-functionlayer 230, including but not limited to the material(s) included in thework-function layer 230, the manner in which the work-function layer 230is deposited, the thickness 250 of the work-function layer 230, and/orthe stress of the work-function layer 230.

Embodiments of work-function layer 230 may include any material known toone of ordinary skill in the art or otherwise suitable for modulatingthe work-function of metallic portion 224. In some embodiments,work-function layer 230 may be a material that has a band gap between4.0 and 5.0 electron-volts (e.g., between 4.0 and 4.5 eV in embodimentswhere transistor 200 b is an n-channel device, or between 4.5 and 5.0 eVin embodiments where transistor 200 b is a p-channel device). In someembodiments, work-function layer 230 may include a metal carbide and/ora metal nitride. For example, in some embodiments, work-function layer230 may include titanium nitride (TiN), titanium carbide (TiC),lanthanum nitride (LaN), lanthanum carbide (LaC), tantalum nitride(TaN), and/or tantalum carbide (TaC). In some embodiments, work-functionlayer 230 may include TiN or TaN alloyed with an oxygen-scavengingmetal, such as TiWN, TiAlN, TiCuN, TaWN, TaAlN, or TaCuN.

In some embodiments, work-function layer 230 may be deposited by aphysical vapor deposition (PVD) technique. For example, work-functionlayer 230 may be deposited by cathodic arc deposition. In someembodiments, the power supplied to the cathode used for cathodic arcdeposition of work-function layer 230 may be less than 3 kW, between 3kW and 9 kW, between 9 kW and 12 kW, between 12 kW and 19 kW, or greaterthan 19 kW. Embodiments are not limited to work-function layersdeposited by cathodic arc deposition or physical vapor deposition. Insome embodiments work-function layer 230 may be formed using anytechnique known to one of ordinary skill in the art or otherwisesuitable for forming a work-function layer.

In some embodiments, work-function layer 230 may include two or moresub-layers. In some embodiments, the sub-layers may include differentmaterials. In some embodiments, the sub-layers may be deposited usingdifferent deposition techniques (e.g., using cathodic arc depositionwith different amounts of power supplied to the cathode duringdeposition of the respective sub-layers). In some embodiments, thesub-layers may have different thicknesses.

As described above, in some embodiments, the strain induced in channel204 may depend on the thickness 250 of work-function layer 230. Thethickness 250 of work-function layer 230 may be, for example, less than100 angstrom (Å), between 100 and 300 Å, between 300 and 500 Å, orgreater than 500 Å.

FIG. 2C shows a transistor 200 c with a gate 240 c that includes one ormore metallic materials (a “metal-gate transistor”), according to someembodiments. In some embodiments, metal-gate transistor 200 c may be aMOSFET formed using a gate-last or gate-first semiconductor fabricationprocess. Many elements of metal gate transistor 200 c shown in FIG. 2Chave already been described above with reference to FIG. 2A. Forbrevity, the description of such elements is not repeated here.

FIG. 2C shows a transistor 200 c with a metallic gate 240 c, accordingto some embodiments. In some embodiments, metal-gate transistor 200 cmay be formed using a gate-first semiconductor fabrication process. Manyelements of metal gate transistor 200 c shown in FIG. 2C have alreadybeen described above with reference to FIGS. 2A and/or 2B. For brevity,the description of such elements is not repeated here.

In the embodiment of FIG. 2C, gate 240 c of metal-gate transistor 200 cincludes a dielectric layer 222 and a metallic portion 224. In someembodiments, gate 240 c of metal-gate transistor 200 c may include or befully or partially covered by a gate-covering layer 220. In someembodiments, portions of dielectric layer 222 may be formed oversubstrate 102 and under metallic portion 224. In some embodiments,portions of dielectric layer 222 may be formed adjacent to the sidewallsof metallic layer 224. In the embodiment of FIG. 2C, channel 204 isstrained (e.g., horizontally and/or vertically, and compressively ortensilely). In some embodiments, gate 240 c applies stress to channel204 which contributes to the strain in channel 204.

FIG. 2D shows a transistor 200 d with a metallic gate 240 d, accordingto some embodiments. In some embodiments, metal-gate transistor 200 dmay be formed using a gate-first semiconductor fabrication process. Manyelements of metal gate transistor 200 d shown in FIG. 2D have alreadybeen described above with reference to FIGS. 2A and/or 2B. For brevity,the description of such elements is not repeated here.

In the embodiment of FIG. 2D, gate 240 d of metal-gate transistor 200 cincludes a dielectric layer 222, a work-function layer 230, and ametallic portion 224. In some embodiments, gate 240 d of metal-gatetransistor 200 d may include or be fully or partially covered by agate-covering layer 220. In some embodiments, portions of dielectriclayer 222 may be formed over substrate 102 and under work-function layer230. In some embodiments, portions of dielectric layer 222 may be formedadjacent to the sidewalls of work-function layer 230 and/or metalliclayer 224. In some embodiments, portions of work-function layer 230 maybe formed over dielectric layer 222 and under metallic portion 224. Insome embodiments, portions of work-function layer 230 may be formedadjacent to the sidewalls of dielectric layer 222 and/or metallicportion 224.

FIGS. 3A and 3B show a flowchart of a method of forming semiconductordevices on an integrated circuit, according to some embodiments. In someembodiments, the method of FIGS. 3A and 3B may be used to formsemiconductor devices of first and second types on an integratedcircuit. The first type of semiconductor device may have a metallic gateand a channel strained in a first manner. The second type ofsemiconductor device may have a metallic gate and a channel strained ina second manner. In some embodiments, for example, the method of FIGS.3A and 3B may be used to form, on an integrated circuit, a metal-gatePFET with a compressively strained channel and a metal-gate NFET with atensilely strained channel.

I. Gate-Last Embodiments of the Method of Forming Semiconductor Devices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on an integrated circuit and as part of a gate-last semiconductorfabrication process, a metal-gate PFET with a compressively strainedchannel and a metal-gate NFET with a tensilely strained channel. In agate-last semiconductor fabrication process, a transistor's source anddrain are formed before the transistor's gate is formed.

In embodiments of FIGS. 3A and 3B that are used as part of a gate-lastsemiconductor fabrication process, step 304 is performed. At step 304,prior to formation of the transistors' metal gates, the transistors'source and drain diffusion regions are formed. In step 304, the sourceand drain diffusion regions of the PFET and the NFET may be formed usingtechniques known to one of ordinary skill in the art or otherwisesuitable for forming drain and source diffusion regions in a gate-lastsemiconductor fabrication process. In some embodiments, ‘dummy’ (e.g.,temporary) gates may be formed (e.g., to facilitate a self-alignment ofa transistor's drain and source regions with the transistor's gate). Forexample, dummy gates may be formed on a substrate of the integratedcircuit in regions corresponding to the gates of the NFET and PFET. Adummy gate may be formed from any material known to one of ordinaryskill in the art or otherwise suitable for forming a temporary gate,including but not limited to polysilicon. In some embodiments, the dummygates may be fully or partially covered by gate-covering layers. Forexample, spacer layers may be formed adjacent to the sidewalls of thedummy gates.

Following the formation of the dummy gates and gate-covering layers,dopants may be implanted into the integrated circuit substrate in thesource and drain regions of the NFET and PFET, the implanted dopants maybe activated, and the dummy gate may be removed. The implantation ofdopants, activation of dopants, and removal of the dummy gate may becarried out using any techniques known to one of ordinary skill in theart or otherwise suitable for implanting dopants, activating dopants, orremoving materials from an integrated circuit. For example, the dopantsmay be activated by annealing the integrated circuit, and the dummy gatemay be removed by etching.

FIG. 4A shows a portion of an integrated circuit 400 a after formationof source and drain diffusion regions in step 304, according to someembodiments. In the embodiment of FIG. 4A, integrated circuit 400 aincludes a substrate 401 (e.g., a bulk silicon substrate). In someembodiments, substrate 401 may be an n-type substrate, region 402 may bea p-well formed in the substrate 401, and region 404 may be a portion ofthe substrate 401. In some embodiments, substrate 401 may be a p-typesubstrate, region 402 may be a portion of substrate 401, and region 404may be an n-well formed in substrate 401. In some embodiments, regions402 and 404 may be fully or partially isolated from each other (e.g., byshallow trench isolation). Embodiments are not limited by the type ofsubstrate (e.g., bulk, SOI, SSDOI, HOI, etc.), the substrate material(e.g., silicon, germanium, etc.), or the type of isolation. In theembodiment of FIG. 4, gate-covering layers 411 are formed in a region ofthe integrated circuit 400 a corresponding to the gate of an NFET, andgate-covering layers 412 are formed in a region of the integratedcircuit 400 a corresponding to the gate of a PFET.

In the embodiment of FIG. 4A, substrate 401 includes source 414 anddrain 416 regions of an NFET, as well as source 418 and drain 419regions of PFET. In some embodiments, the source and drain diffusionregions (414, 416) of the NFET may be implanted (“doped”) or heavilyimplanted (“heavily doped”) with n-type dopants. In some embodiments,the source and drain diffusion regions (418, 419) of the PFET may beimplanted (“doped”) or heavily implanted (“heavily doped”) with p-typedopants. Although the drains and sources illustrated in FIG. 4 are notraised, embodiments are not limited in this regard. In some embodiments,one or more of the NFET source 414, NFET drain 416, PFET source 418, orPFET drain 419 may be raised.

II. Gate-First Embodiments of the Method of Forming SemiconductorDevices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on an integrated circuit and as part of a gate-first semiconductorfabrication process, a metal-gate PFET with a compressively strainedchannel and a metal-gate NFET with a tensilely strained channel. In agate-first semiconductor fabrication process, a transistor's gate isformed before the transistor's source and drain diffusion regions areformed.

In embodiments of FIGS. 3A and 3B that are used as part of a gate-firstsemiconductor fabrication process, step 336 is performed. At step 336,subsequent to formation of the transistors' metal gates, thetransistors' source and drain diffusion regions are formed usingtechniques known to one of ordinary skill in the art or otherwisesuitable for forming drain and source diffusion regions in a gate-firstsemiconductor fabrication process. In some embodiments of step 336,source and drain diffusion regions may be formed by implanting dopantsinto the integrated circuit substrate in the source and drain regions ofthe NFET and PFET, and by activating the implanted dopants. Theimplantation and activation of dopants may be carried out using anytechniques known to one of ordinary skill in the art or otherwisesuitable for implanting and activating dopants.

In some embodiments of step 336, the gates may be fully or partiallycovered by gate-covering layers. In some embodiments, gate-coveringlayers 413 and 415 may include a same material deposited in a same stepof an integrated circuit fabrication process.

FIG. 4B shows a portion of an integrated circuit 400 b after formationof source and drain diffusion regions in step 336, according to someembodiments. Many elements of integrated circuit 400 b shown in FIG. 4Bhave already been described above with reference to FIG. 4A. Forbrevity, descriptions of such elements are not repeated here.

In the embodiment of FIG. 4B, NFET 405 and PFET 407 include gates 406and 408, respectively. In some embodiments, either gate or both gatesmay include a metallic material. In some embodiments, NFET gate 406 maybe fully or partially covered by a gate-covering layer 413, and NFETgate 408 may be fully or partially covered by a gate-covering layer 415.

III. An Embodiment of a Method of Forming Semiconductor Devices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on an integrated circuit and as part of a gate-last process or agate-first process, a metal-gate PFET with a compressively strainedchannel and a metal-gate NFET with a tensilely strained channel, whereinthe metal gates' metallic portions include the same material(s), andwherein the PFET gate or the NFET gate has a work-function layer, butthe other type of gate does not. In some embodiments, formation of sucha metal-gate PFET and a metal-gate NFET may include steps 306, 308, 312,314, and 333 of the method illustrated in FIGS. 3A and 3B.

At step 306, a dielectric layer is deposited over portions of theintegrated circuit corresponding to the PFET and the NFET. Thedielectric layer may be deposited using any technique known to one ofordinary skill in the art or otherwise suitable for depositing adielectric material. The dielectric layer may include, for example, ahigh-k dielectric material such as silicon dioxide (SiO₂).

At step 308, a first metallic layer is deposited over the portions ofthe integrated circuit corresponding to the PFET and the NFET. The firstmetallic layer may be deposited using any technique known to one ofordinary skill in the art or otherwise suitable for depositing ametallic material, including but not limited to physical vapordeposition (e.g., cathodic arc deposition). In some embodiments, thefirst metallic layer may function as a work-function layer. The firstmetallic layer may include, for example, a metal carbide and/or a metalnitride.

At step 312, the first metallic layer (e.g., work-function layer) isremoved from a portion of the integrated circuit corresponding to thePFET or the NFET. The first metallic layer may be removed from theselected portion of the integrated circuit using any technique known toone of ordinary skill in the art or otherwise suitable for selectivelyremoving a metallic material from an integrated circuit, including butnot limited to photolithographic patterning, dry etching, wet etching,reactive ion etching, isotropic etching, anisotropic etching, etc.

At step 314, a second metallic layer is deposited over the portions ofthe integrated circuit corresponding to the PFET and the NFET. Thesecond metallic layer may be deposited using any technique known to oneof ordinary skill in the art or otherwise suitable for depositing ametallic material. In some embodiments, the second metallic layer mayinclude material(s) suitable for use as a metallic portion of a metalgate. The second metallic layer may include, for example, aluminum (Al),tungsten (W), and/or copper (Cu).

At step 333, portions of the dielectric layer, metallic layers, and/orgate-covering layers which do not correspond to the gates of the PFETand NFET may be removed, such that the remaining gates of the PFET andthe NFET remain. Portions of one or more of these layers may be removedusing any technique known to one of ordinary skill in the art orotherwise suitable for selectively removing such materials from anintegrated circuit. For example, photolithographic patterning, dryetching, wet etching, reactive ion etching, isotropic etching, oranisotropic etching may be used to remove portions of these layers fromareas of the integrated circuit other than the gates of the PFET andNFET.

FIG. 5A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last process using steps 304,306, 308, 312, 314, and 333 of the method illustrated in FIGS. 3A and3B, according to some embodiments. Many elements of integrated circuit400 a shown in FIG. 5A have already been described above with referenceto FIG. 4A. For brevity, descriptions of such elements are not repeatedhere.

In the embodiment of FIG. 5A, NFET 405 includes a gate 406, a sourcediffusion region 414, a drain diffusion region 416, a channel region409, and a body. The body is formed in region 402 of substrate 401.Channel 409 may occupy portions of substrate 401 between source 414 anddrain 416 under gate 406. In some embodiments, channel 409 may betensilely strained. In the embodiment of FIG. 5A, gate 406 of metal-gateNFET 405 includes dielectric layer 420 (deposited in step 306 of themethod of FIGS. 3A and 3B) and second metallic layer 422 (deposited instep 314). Second metallic layer 422 may function as a metallic portionof gate 406. In some embodiments, gate 406 may apply tensile stress tochannel 409. In some embodiments, gate 406 may also include or be fullyor partially covered by gate-covering layer 411. In some embodiments,gate-covering layer 411 may be removed during fabrication of theintegrated circuit (e.g., during step 333 of the method of FIGS. 3A and3B).

In the embodiment of FIG. 5A, PFET 407 includes a gate 408, a sourcediffusion region 418, a drain diffusion region 419, a channel region410, and body. The body is formed in region 404 of substrate 401.Channel 410 may occupy portions of substrate 401 between source 418 anddrain 419 under gate 408. In some embodiments, channel 410 may becompressively strained. In the embodiment of FIG. 5A, gate 408 ofmetal-gate PFET 407 includes dielectric layer 420 (deposited in step 306of the method of FIGS. 3A and 3B), first metallic layer 421 (depositedin step 308), and second metallic layer 422 (deposited in step 314).First metallic layer 421 may function as a work-function layer, andsecond metallic layer 422 may function as a metallic portion of gate408. In some embodiments, gate 408 may apply compressive stress tochannel 410. In some embodiments, gate 408 may include or be fully orpartially covered by gate-covering layer 412. In some embodiments,gate-covering layer 412 may be removed during fabrication of theintegrated circuit (e.g., during step 333 of the method of FIGS. 3A and3B).

FIG. 5B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 312, 314, 333, and 336 of the method illustrated in FIGS. 3Aand 3B, according to some embodiments. The elements of integratedcircuit 400 b shown in FIG. 5B have already been described above withreference to FIGS. 4B and 5A. For brevity, descriptions of such elementsare not repeated here.

Thus, each integrated circuit (400 a, 400 b) of FIGS. 5A and 5B includesa metal-gate NFET with a tensilely-strained channel and a metal gatePFET with a compressively-strained channel. In some embodiments, thegates of the metal-gate NFET and PFET collectively include three orfewer metallic materials. For example, in some embodiments, the gate ofthe metal-gate NFET includes one metallic material (second metalliclayer 422), and the gate of metal-gate PFET includes two metallicmaterials (first metallic layer 421 and second metallic layer 422).Also, in some embodiments, the gates of the metal-gate NFET and PFETshare a metallic material (e.g., second metallic layer 422). In someembodiments, the metallic material shared by the gates of the NFET andPFET may be deposited on the integrated circuit in a same processingstep of an integrated circuit fabrication process.

IV. Another Embodiment of a Method of Forming Semiconductor Devices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on the integrated circuit and as part of a gate-last process or agate-first process, a metal-gate PFET with a compressively strainedchannel and a metal-gate NFET with a tensilely strained channel, whereinthe metallic portions of the metal gates include different materials andthe PFET gate or the NFET gate has a work-function layer, but the othertype of gate does not. In some embodiments, formation of such ametal-gate PFET and metal-gate NFET may include steps 306, 308, 312,314, 318, 320, 323, and 333 of the method illustrated in FIGS. 3A and3B.

Embodiments of steps 306, 308, 312, 314, and 333 suitable for use informing such a metal-gate PFET and metal-gate NFET are described abovein Section III with reference to the method of FIGS. 3A and 3B. In theinterest of brevity, descriptions of these steps are not repeated here.

At step 318, the second metallic layer is removed from a portion of theintegrated circuit corresponding to the PFET or the NFET. The secondmetallic layer may be removed from the selected portion of theintegrated circuit using any technique known to one of ordinary skill inthe art or otherwise suitable for selectively removing a metallicmaterial from an integrated circuit, including but not limited tophotolithographic patterning, dry etching, wet etching, reactive ionetching, isotropic etching, anisotropic etching, etc.

At step 320, a third metallic layer is deposited over the portions ofthe integrated circuit corresponding to the PFET and the NFET. The thirdmetallic layer may be deposited using any technique known to one ofordinary skill in the art or otherwise suitable for depositing ametallic material. In some embodiments, the second metallic layer mayfunction as a metallic layer of a metal gate. The third metallic layermay include, for example, aluminum (Al), tungsten (W), and/or copper(Cu).

At step 323, the third metallic layer is removed from a portion of theintegrated circuit corresponding to the PFET or the NFET, such that thesecond metallic layer remains over the portion of the integrated circuitcorresponding to one of the PFET or the NFET, and the third metalliclayer remains over the portion of the integrated circuit correspondingto the other of the PFET or NFET. The third metallic layer may beremoved from the selected portion of the integrated circuit using anytechnique known to one of ordinary skill in the art or otherwisesuitable for selectively removing a metallic material from an integratedcircuit, including but not limited to photolithographic patterning, dryetching, wet etching, reactive ion etching, isotropic etching,anisotropic etching, etc.

FIG. 6A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 312, 314, 318, 320, 323, and 333 of themethod illustrated in FIGS. 3A and 3B, according to some embodiments.Many elements of integrated circuit 400 a shown in FIG. 6A have alreadybeen described above with reference to FIGS. 4A and/or 5A. For brevity,descriptions of such elements are not repeated here.

In the embodiment of FIG. 6A, gate 406 of metal-gate NFET 405 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B) and second metallic layer 422 (deposited in step 314). Secondmetallic layer 422 may function as a metallic portion of gate 406. Insome embodiments, gate 406 may apply tensile stress to channel 409. Insome embodiments, gate 406 may include or be fully or partially coveredby gate-covering layer 411. In some embodiments, gate-covering layer 411may be removed during fabrication of the integrated circuit (e.g.,during step 333 of the method of FIGS. 3A and 3B).

In the embodiment of FIG. 6A, gate 408 of metal-gate PFET 407 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), first metallic layer 421 (deposited in step 308), and thirdmetallic layer 423 (deposited in step 320). First metallic layer 421 mayfunction as a work-function layer, and third metallic layer 423 mayfunction as a metallic portion of gate 408. In some embodiments, gate408 may apply compressive stress to channel 410. In some embodiments,gate 408 may include or be fully or partially covered by gate-coveringlayer 412. In some embodiments, gate-covering layer 412 may be removedduring fabrication of the integrated circuit (e.g., during step 333 ofthe method of FIGS. 3A and 3B).

FIG. 6B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 312, 314, 318, 320, 323, 333, and 336 of the methodillustrated in FIGS. 3A and 3B, according to some embodiments. Theelements of integrated circuit 400 b shown in FIG. 6B have already beendescribed above with reference to FIGS. 4B and 6A. For brevity,descriptions of such elements are not repeated here.

Thus, each integrated circuit (400 a, 400 b) of FIGS. 6A and 6B includesa metal-gate NFET with a tensilely-strained channel and a metal-gatePFET with a compressively-strained channel. In some embodiments, thegates of the metal-gate NFET and PFET collectively include three orfewer metallic materials. For example, in some embodiments, the gate ofthe metal-gate NFET includes one metallic material (second metalliclayer 422), and the gate of metal-gate PFET includes two metallicmaterials (first metallic layer 421 and third metallic layer 423).

V. Another Embodiment of a Method of Forming Semiconductor Devices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on the integrated circuit and as part of a gate-last or a gate-firstprocess, a metal-gate PFET with a compressively strained channel and ametal-gate NFET with a tensilely strained channel, wherein the metalgates have different work-function layers, and wherein the metal gates'metallic portions include the same materials. In some embodiments,formation of such a metal-gate PFET and metal-gate NFET may includesteps 306, 308, 312, 314, 318, 320, and 333 of the method illustrated inFIGS. 3A and 3B.

Embodiments of steps 306, 308, and 312 suitable for use in forming sucha metal-gate PFET and metal-gate NFET are described above in Section IIIwith reference to the method of FIGS. 3A and 3B. Embodiments of steps320 and 333 suitable for use in forming such a metal-gate PFET andmetal-gate NFET are described above in Sections III and IV,respectively, with reference to the method of FIGS. 3A and 3B. In theinterest of brevity, descriptions of these steps are not repeated here.

At step 314, a second metallic layer is deposited over the portions ofthe integrated circuit corresponding to the PFET and the NFET. Thesecond metallic layer may be deposited using any technique known to oneof ordinary skill in the art or otherwise suitable for depositing ametallic material, including but not limited to physical vapordeposition (e.g., cathodic arch deposition). In some embodiments, thesecond metallic layer may function as a work-function layer. The secondmetallic layer may include, for example, a metal carbide and/or a metalnitride.

At step 318, the second metallic layer (e.g., work-function layer) isremoved from a portion of the integrated circuit corresponding to thePFET or the NFET, such that the first metallic layer remains over theportion of the integrated circuit corresponding to one of the PFET orthe NFET, and the second metallic layer remains over the portion of theintegrated circuit corresponding to the other of the PFET or NFET. Thesecond metallic layer may be removed from the selected portion of theintegrated circuit using any technique known to one of ordinary skill inthe art or otherwise suitable for selectively removing a metallicmaterial from an integrated circuit, including but not limited tophotolithographic patterning, dry etching, wet etching, reactive ionetching, isotropic etching, anisotropic etching, etc.

FIG. 7A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 312, 314, 318, 320, and 333 of the methodillustrated in FIGS. 3A and 3B, according to some embodiments. Manyelements of integrated circuit 400 a shown in FIG. 7A have already beendescribed above with reference to FIGS. 4A and 5A. For brevity,descriptions of such elements are not repeated here.

In the embodiment of FIG. 7A, gate 406 of metal-gate NFET 405 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), second metallic layer 422 (deposited in step 314), and thirdmetallic layer 423 (deposited in step 320). Second metallic layer 422may function as a work-function layer of gate 406. Third metallic layer423 may function as a metallic portion of gate 406. In some embodiments,gate 406 may apply tensile stress to channel 409. In some embodiments,gate 406 may include or be fully or partially covered by gate-coveringlayer 411.

In the embodiment of FIG. 7A, gate 408 of metal-gate PFET 407 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), first metallic layer 421 (deposited in step 308), and thirdmetallic layer 423 (deposited in step 320). First metallic layer 421 mayfunction as a work-function layer, and third metallic layer 423 mayfunction as a metallic portion of gate 408. In some embodiments, gate408 may apply compressive stress to channel 410. In some embodiments,gate 408 may include or be fully or partially covered by gate-coveringlayer 412.

FIG. 7B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 312, 314, 318, 320, 333, and 336 of the method illustrated inFIGS. 3A and 3B, according to some embodiments. The elements ofintegrated circuit 400 b shown in FIG. 7B have already been describedabove with reference to FIGS. 4B and 7A. For brevity, descriptions ofsuch elements are not repeated here.

Thus, each integrated circuit (400 a, 400 b) of FIGS. 7A and 7B includesa metal-gate NFET with a tensilely-strained channel and a metal gatePFET with a compressively-strained channel. In some embodiments, thegates of the metal-gate NFET and PFET collectively include three orfewer metallic materials. For example, in some embodiments, the gate ofthe metal-gate NFET includes second metallic layer 422, the gate ofmetal-gate PFET includes first metallic layer 421, and the gates of bothFETs include third metallic layer 423. Also, in some embodiments, thegates of the metal-gate NFET and PFET share a metallic material (e.g.,third metallic layer 423). In some embodiments, the metallic materialshared by the gates of the NFET and PFET may be deposited on theintegrated circuit in a same processing step of an integrated circuitfabrication process.

VI. Another Embodiment of a Method of Forming Semiconductor Devices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on an integrated circuit and as part of a gate-last process or agate-first process, a metal-gate PFET with a compressively strainedchannel and a metal-gate NFET with a tensilely strained channel, whereinthe metal gates have the same work-function layers and differentmetallic layers. In some embodiments, formation of such a metal-gatePFET and metal-gate NFET may include steps 306, 308, 324, 328, 330, 332,and 333 of the method illustrated in FIGS. 3A and 3B.

Embodiments of steps 306, 308, and 333 suitable for use in forming sucha metal-gate PFET and metal-gate NFET are described above in Section IIIwith reference to the method of FIGS. 3A and 3B. In the interest ofbrevity, descriptions of these steps are not repeated here.

At step 324, a second metallic layer is deposited over the portions ofthe integrated circuit corresponding to the PFET and the NFET. Thesecond metallic layer may be deposited using any technique known to oneof ordinary skill in the art or otherwise suitable for depositing ametallic material. In some embodiments, the second metallic layer mayfunction as a metallic portion of a metal gate. The second metalliclayer may include, for example, aluminum (Al), tungsten (W), and/orcopper (Cu).

At step 328, the second metallic layer is removed from a portion of theintegrated circuit corresponding to the PFET or the NFET. The secondmetallic layer may be removed from the selected portion of theintegrated circuit using any technique known to one of ordinary skill inthe art or otherwise suitable for selectively removing a metallicmaterial from an integrated circuit, including but not limited tophotolithographic patterning, dry etching, wet etching, reactive ionetching, isotropic etching, anisotropic etching, etc.

At step 330, a third metallic layer is deposited over the portions ofthe integrated circuit corresponding to the PFET and the NFET. The thirdmetallic layer may be deposited using any technique known to one ofordinary skill in the art or otherwise suitable for depositing ametallic material. In some embodiments, the third metallic layer mayfunction as a metallic layer of a metal gate. The second metallic layermay include, for example, aluminum (Al), tungsten (W), and/or copper(Cu).

At step 332, the third metallic layer is removed from a portion of theintegrated circuit corresponding to the PFET or the NFET, such that thesecond metallic layer remains over the portion of the integrated circuitcorresponding to one of the PFET or the NFET, and the third metalliclayer remains over the portion of the integrated circuit correspondingto the other of the PFET or NFET. The third metallic layer may beremoved from the selected portion of the integrated circuit using anytechnique known to one of ordinary skill in the art or otherwisesuitable for selectively removing a metallic material from an integratedcircuit, including but not limited to photolithographic patterning, dryetching, wet etching, reactive ion etching, isotropic etching,anisotropic etching, etc.

FIG. 8A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 324, 328, 330, 332, and 333 of the methodillustrated in FIGS. 3A and 3B, according to some embodiments. Manyelements of integrated circuit 400 a shown in FIG. 8A have already beendescribed above with reference to FIGS. 4A and 5A. For brevity,descriptions of such elements are not repeated here.

In the embodiment of FIG. 8A, gate 406 of metal-gate NFET 405 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), first metallic layer 421 (deposited in step 308), and thirdmetallic layer 423 (deposited in step 330). First metallic layer 421 mayfunction as a work-function layer, and third metallic layer 423 mayfunction as a metallic portion of gate 406. In some embodiments, gate406 may apply tensile stress to channel 409. In some embodiments, gate406 may include or be fully or partially covered by gate-covering layer411.

In the embodiment of FIG. 8A, gate 408 of metal-gate PFET 407 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), first metallic layer 421 (deposited in step 308), and secondmetallic layer 422 (deposited in step 324). First metallic layer 421 mayfunction as a work-function layer, and second metallic layer 422 mayfunction as a metallic portion of gate 408. In some embodiments, gate408 may apply compressive stress to channel 410. In some embodiments,gate 408 may include or be fully or partially covered by gate-coveringlayer 412.

FIG. 8B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 324, 328, 330, 332, 333, and 336 of the method illustrated inFIGS. 3A and 3B, according to some embodiments. The elements ofintegrated circuit 400 b shown in FIG. 8B have already been describedabove with reference to FIGS. 4B and 8A. For brevity, descriptions ofsuch elements are not repeated here.

Thus, each integrated circuit (400 a, 400 b) of FIGS. 8A and 8B includesa metal-gate NFET with a tensilely-strained channel and a metal gatePFET with a compressively-strained channel. In some embodiments, thegates of the metal-gate NFET and PFET collectively include three orfewer metallic materials. For example, in some embodiments, the gate ofthe metal-gate NFET includes third metallic layer 423 (a metallicmaterial), the gate of metal-gate PFET includes second metallic layer422 (another metallic material), and the gates of both FETs includefirst metallic layer 421 (a third metallic material). Also, in someembodiments, the gates of the metal-gate NFET and PFET share a metallicmaterial (e.g., first metallic layer 421). In some embodiments, themetallic material shared by the gates of the NFET and PFET may bedeposited on the integrated circuit in a same processing step of anintegrated circuit fabrication process.

VII. Another Embodiment of a Method of Forming Semiconductor Devices

In some embodiments, the method of FIGS. 3A and 3B may be used to form,on the integrated circuit and as part of a gate-last process or agate-first process, a metal-gate PFET with a compressively strainedchannel and a metal-gate NFET with a tensilely strained channel, whereinthe metal gates have the same work-function layers and the same metalliclayers. In some embodiments, formation of such a metal-gate PFET andmetal-gate NFET may include steps 306, 308, 324, and 333 of the methodillustrated in FIGS. 3A and 3B.

Embodiments of steps 306, 308, 324, and 333 suitable for use in formingsuch a metal-gate PFET and metal-gate NFET are described above inSection VI of the method of FIGS. 3A and 3B. In the interest of brevity,descriptions of these steps are not repeated here.

FIG. 9A shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 a as part of a gate-last fabrication processusing steps 304, 306, 308, 324, and 333 of the method illustrated inFIGS. 3A and 3B, according to some embodiments. Many elements ofintegrated circuit 400 a shown in FIG. 9A have already been describedabove with reference to FIGS. 4A and 5A. For brevity, descriptions ofsuch elements are not repeated here.

In the embodiment of FIG. 9A, gate 406 of metal-gate NFET 405 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), first metallic layer 421 (deposited in step 308), and secondmetallic layer 422 (deposited in step 324). First metallic layer 421 mayfunction as a work-function layer, and second metallic layer 422 mayfunction as a metallic portion of gate 406. In some embodiments, gate406 may apply tensile stress to channel 409. In some embodiments, gate406 may include or be fully or partially covered by gate-covering layer411.

In the embodiment of FIG. 9A, gate 408 of metal-gate PFET 407 includesdielectric layer 420 (deposited in step 306 of the method of FIGS. 3Aand 3B), first metallic layer 421 (deposited in step 308), and secondmetallic layer 422 (deposited in step 324). First metallic layer 421 mayfunction as a work-function layer, and second metallic layer 422 mayfunction as a metallic portion of gate 408. In some embodiments, gate408 may apply compressive stress to channel 410. In some embodiments,gate 408 may include or be fully or partially covered by gate-coveringlayer 412.

FIG. 9B shows a metal-gate PFET and a metal-gate NFET formed on anintegrated circuit 400 b as part of a gate-first process using steps306, 308, 324, 333, and 336 of the method illustrated in FIGS. 3A and3B, according to some embodiments. The elements of integrated circuit400 b shown in FIG. 9B have already been described above with referenceto FIGS. 4B and 9A. For brevity, descriptions of such elements are notrepeated here.

Thus, each integrated circuit (400 a, 400 b) of FIGS. 9A and 9B includesa metal-gate NFET with a tensilely-strained channel and a metal gatePFET with a compressively-strained channel. In some embodiments, thegates of the metal-gate NFET and PFET collectively include three orfewer metallic materials. For example, in some embodiments, the gates ofthe NFET and PFET collectively include first metallic layer 421 (ametallic material) and second metallic layer 422 (a second metallicmaterial). Also, in some embodiments, the gates of the metal-gate NFETand PFET share a metallic material (e.g., first metallic layer 421and/or second metallic layer 422). In some embodiments, a metallicmaterial shared by the gates of the NFET and PFET may be deposited onthe integrated circuit in a same processing step of an integratedcircuit fabrication process.

As illustrated by the examples above, embodiments of the method of FIGS.3A and 3B may include subsets of the steps illustrated in FIGS. 3A and3B. Some embodiments may include a single step illustrated in FIGS. 3Aand 3B. In embodiments of the method that include two or more of thesteps illustrated in FIGS. 3A and 3B, the steps are not necessarilyperformed in the order shown in FIGS. 3A and 3B. For example, in someembodiments, steps 320 and 323 could be performed prior to steps 314 and318.

Although the foregoing disclosure refers to NFETs and PFETs as examplesof semiconductor devices, embodiments are not limited in this regard.The techniques described herein may be used to enhance the mobility ofcharge carriers in any semiconductor device known to one of ordinaryskill in the art or otherwise suitable for operating withenhanced-mobility charge carriers, including but not limited to anyelectrical device, MEMS (micro-electromechanical system) device,optoelectronic device, etc.

In some embodiments, a work-function layer comprising TiN with athickness of 400 angstrom (Å), deposited by cathodic arc deposition withcathode supplied at 12 kW DC, may perform well as a work-function layerfor a metal-gate PFET, and provide a good balance between the stress anddensity of the work-function layer.

Embodiments described in the present disclosure may be included in anyelectronic device, including but not limited to a microprocessor, amobile electronic device, a mobile phone, a smart phone, a tabletcomputer, a laptop computer, a desktop computer, or a server.

Terms used herein to describe positioning relationships of structuralelements, such as “over,” “under,” “beside,” and “adjacent to,” shouldnot be construed as requiring the structural elements to be in contactwith each other or directly related (e.g., “over” should not beconstrued to mean “directly over” or to require that no other structuresintervene between structure A and structure B when structure A isdescribed as being “over” structure B), even where some or allembodiments of the structural elements illustrated in the Figures showthe structural elements being in contact with each other and/orpositioned without any structures intervening between them.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

1. A method, comprising: forming first and second polysilicon gates on a semiconductor substrate; forming p-type source and drain regions in the semiconductor substrate, using the first polysilicon gate as a mask; forming n-type source and drain regions in the semiconductor substrate, using the second polysilicon gate as a mask; forming spacers in contact with sidewalls of the first and second polysilicon gates; replacing the first polysilicon gate with a first metal gate structure including a gate dielectric, a first metallic layer, and a second metallic layer, the first metal gate structure applying compressive stress to a channel region of the semiconductor substrate located between the p-type source and drain regions; and replacing the second polysilicon gate with a second metal gate structure including the gate dielectric, the second metallic layer, and a third metallic layer, the second metal gate structure applying tensile stress to a channel region of the semiconductor substrate located between the n-type source and drain regions.
 2. The method of claim 1, further comprising forming raised source regions and raised drain regions in contact with the source and drain regions, respectively.
 3. The method of claim 1 wherein one or more of the metal gate structures includes a work function layer.
 4. The method of claim 3 wherein the work function layer is the second metallic layer.
 5. The method of claim 3 wherein the work function layer is the first metallic layer.
 6. The method of claim 1 wherein one or more of the metallic layers is formed by physical vapor deposition (PVD).
 7. The method of claim 6 wherein the physical vapor deposition includes cathodic arc deposition.
 8. The method of claim 1 wherein replacing the first and second poly silicon gates entails conformally depositing the gate dielectric and the first and second metallic layers between the spacers.
 9. The method of claim 1 wherein the spacers have straight vertical profiles.
 10. The method of claim 1 wherein replacing the first and second polysilicon gates with respective metal gate structures includes forming the first metallic layer as a portion of both the first and second metal gate structures; removing the first metallic layer from the second metal gate structure; forming the second metallic layer as a portion of both the first and second metal gate structures; forming the third metallic layer as a portion of both the first and second metal gate structures; and removing the third metallic layer from the first metal gate structure.
 11. A method, comprising: forming first and second polysilicon gates on a semiconductor substrate; forming p-type source and drain regions in the semiconductor substrate, using the first polysilicon gate as a mask; forming n-type source and drain regions in the semiconductor substrate, using the second polysilicon gate as a mask; forming a first spacer in contact with sidewalls of the first polysilicon gate, the first spacer applying compressive stress to a channel region of the semiconductor substrate located between the p-type source and drain regions; forming a second spacer in contact with sidewalls of the second polysilicon gate, the second spacer applying tensile stress to a channel region of the semiconductor substrate located between the n-type source and drain regions; and replacing each of the first and second polysilicon gates with a metal gate structure including a gate dielectric, a first metallic layer, and a second metallic layer.
 12. A method, comprising: forming p-type source and drain regions in a semiconductor substrate; forming n-type source and drain regions in the semiconductor substrate; forming first and second metal gate structures over the p-type and n-type source and drain regions, respectively, the first and second metal gate structures including a gate dielectric layer, a first metallic layer, and a second metallic layer; forming a first continuous spacer wrapping around four vertical sidewalls and a top of the first metal gate structure, the first continuous spacer applying compressive stress to a channel region of the semiconductor substrate located between the p-type source and drain regions; and forming a second continuous spacer wrapping around four vertical sidewalls and a top of the second metal gate structure, the second continuous spacer applying tensile stress to a channel region of the semiconductor substrate located between the n-type source and drain regions.
 13. A method, comprising: forming p-type source and drain regions in a semiconductor substrate; forming n-type source and drain regions in the semiconductor substrate; forming a first metal gate structure over the p-type source and drain regions, the first metal gate structure including a gate dielectric layer, a first metallic layer, and a second metallic layer, the first metal gate structure applying compressive stress to a channel region of the semiconductor substrate located between the p-type source and drain regions; forming a second metal gate structure over the n-type source and drain regions, the second metal gate structure including a gate dielectric layer, the second metallic layer, and a third metallic layer, the second metal gate structure applying tensile stress to a channel region of the semiconductor substrate located between the n-type source and drain regions; forming a continuous spacer over each metal gate structure, the continuous spacer wrapping around four vertical sidewalls and a top of the respective metal gate structure.
 14. The method of claim 13 wherein forming the first and second metal gate structures includes forming the first metallic layer as a portion of both the first and second metal gate structures; removing the first metallic layer from the second metal gate structure; forming the second metallic layer as a portion of both the first and second metal gate structures; forming the third metallic layer as a portion of both the first and second metal gate structures; and removing the third metallic layer from the first metal gate structure.
 15. The method of claim 13 wherein the metallic layers include a work function material.
 16. The method of claim 15 wherein, when the source and drain regions are n-type, the work function material has a band gap in the range of 4.0-5.0 eV.
 17. The method of claim 15 wherein, when the source and drain regions are p-type, the work function material has a band gap in the range of 4.5-5.0 eV.
 18. The method of claim 15 wherein the work function material includes one or more of TiN, TiC, LaN, LaC, TaN, TaC, TiWN, TiAlN, TiCuN, TaWN, TaAlN, or TaCuN.
 19. The method of claim 15 wherein the work function material has a thickness in the range of 100-500 Angstroms.
 20. The method of claim 13 wherein the stress applied to the channel regions causes a strain in the channel regions in the range of 100-500 MPa. 